已知方程组$\left\{\begin{array}{c}ax-by=4\\ 2x+3y=4\end{array}\right.$与$\left\{\begin{array}{c}ax+by=2\\ 4x-3y=2\end{array}\right.$的解相同，那么a+b=____ 

根据题意把文章补充完整

All photos are made with a camera. In many ways, a camera works like a human eye.Like the eye, it takes in rays of light reflected from an object. It then focuses the rays into ____ image（影像）.Older cameras record the imagine on film.

  Newer digital cameras record the imagine on an electronic storage device（存储装置）such as a memory card. Once the image is permanent, it can ____(see) and enjoyed by many people.

  The photographic process for older cameras has five steps to it:Finding a subject;Focusing

____ the subject;Exposing the film;Developing the film,and making the print. Many professional photographers, people____take pictures for a living, do all five steps ____(they). They use a room called a darkroom. It's “dark”so that light doesn't ruin in the negatives(底片）.

  Photography makes our lives richer in many ways. Through photos we can learn about other

____（part) of the world. We can see how people live today. We can also see how people lived in the mid-1800's, around the time when the camera ____(invent). Special cameras can capture images in places most human beings can't go, like the moon.____ ,best of all, photographs remind us of the special people in our lives. Millions of people the world over use camera____ (take)pictures of their family,friends,and special____(celebrate). For them, the photographs they take are priceless.

第二节

先估算，再计算．

18×503≈____

18×503＝____